Recent technological advancements in light-emitting diode (LED) design have been a boon to the lighting industry. With their high overall luminous efficacy and flexibility for achieving various light patterns, LED-based luminaires are increasingly being used in signage, advertising, display lighting, and backlit lighting applications. LED-based luminaires are also replacing the traditional incandescent or fluorescent lighting fixtures to become the mainstream lighting architecture.
Due to its natural lighting characteristics, white light is the preferred choice for lighting. An important consideration for LED-based luminaires used for ambient lighting is the need to produce natural white light. White light can be generated by mixing the light emitted from different colour LEDs.
Various standards have been proposed to characterize the spectral content of light. One way to characterize light emitted by a test light source is to compare it with the light radiated by a black body and identify the temperature of the black body at which its perceived colour best matches the perceived colour of the test light source. That temperature is called correlated colour temperature (CCT) and is usually measured in Kelvin (K). The higher the CCT, the bluer, or cooler the light appears. The lower the CCT, the redder, or warmer the light appears. An incandescent light bulb has a CCT of approximately 2854 K, and fluorescent lamps can have CCTs in the range of approximately 3200 K to 6500 K.
Furthermore the properties of light can be characterized in terms of luminous flux and chromaticity. Luminous flux is used to define the measurable amount of light and chromaticity is used to define the perceived colour impression of light, irrespective of its perceived brightness. Chromaticity and luminous flux are measured in units according to standards of the Commission Internationale de l'Eclairage (CIE). The CIE chromaticity standards define hue and saturation of light based on chromaticity coordinates that specify a position in a chromaticity diagram. The chromaticity coordinates of light are derived from tristimulus values and expressed by the ratio of the tristimulus values to their sum; i.e. x=X/(X+Y+Z), y=Y/(X+Y+Z), z=Z/(X+Y+Z), where x, y and z are the chromaticity coordinates and X, Y, and Z the tristimulus values. Because x+y+z=1, it is only necessary to specify two chromaticity coordinates such as x and y, for example. Any CCT value can be transformed into corresponding chromaticity coordinates.
In spite of their success, LED-based luminaires can be affected by a number of parameters in a complex way. Chromaticity and luminous flux output of LEDs can greatly depend on junction temperature and drive current as well as device aging effects that result in efficacy degradation over time, which can have undesirable effects on the CCT and more generally the chromaticity of the emitted light.
Ignoring temperature dependencies, the amount of light emitted by an LED is proportional to its instantaneous forward current. If the LEDs are pulsed at a rate greater than about 300 Hz, the human visual system perceives a time-averaged amount of light as opposed to individual pulses. As a result, luminaire dimming can be achieved by varying the amount of time-averaged forward current, using such techniques as pulse width modulation (PWM) or pulse code modulation (PCM). However, changes in the average forward current can affect the junction temperature of the LED, which can alter the spectral power distribution and in consequence the CCT or chromaticity and luminous flux of the light emitted by the LED. The compensation of this effect can become complex when various coloured LEDs are used to generate mixed light of a desired chromaticity. As discussed by M. Dyble, in “Impact of Dimming White LEDs: Chromaticity Shifts Due to Different Dimming Methods,” Fifth International Conference on Solid State Lighting, Bellingham, Wash.; SPIE Vol. 5941, 2005, colour appearance of the resultant mixed light can shift unacceptably when dimming, as the spectral power distribution of the individual LEDs can change.
LED junction temperature variations can also cause undesired effects on the spectral power distribution of the resultant output light. Variations in junction temperature not only can reduce the luminous flux output, but can also cause undesirable variations in the CCT of the mixed light. Overheating can also reduce the life span of LEDs.
In order to overcome these limitations, various methods for generating natural white light have been proposed. U.S. Pat. No. 6,448,550 to Nishimura teaches a solid-state illumination device having a plurality of LEDs of different colours using optical feedback. Light from the LEDs is measured by photosensitive sensors mounted in close proximity with LEDs and compared with a reference set of responses to a previously measured spectral power distribution. The amount of variation between the sensor responses to the light from the LEDs and the previously measured spectral power distribution is used as a basis for adjusting the current to the LEDs in order to maintain the light from the LEDs as close as possible to the pre-determined spectral power distribution. While the Nishimura reference provides an effective way to achieve control of the spectral power distribution of the output light with any desired colour property, it does not consider maintaining colour stability over the life of the LEDs and at different operating conditions, including dimming.
U.S. Pat. No. 6,507,159 to Muthu discloses a control method and system for an LED-based luminaire having a plurality of red, green and blue light LEDs for generating a desired light by colour mixing. Muthu seeks to alleviate the unwanted variations in the luminous flux output and CCT of the desired light by providing a control system with a feedback system including filtered photodiodes, a mathematical transformation for determining tristimulus values of the LEDs, and a reference-tracking controller for resolving the difference between the feedback tristimulus values and the desired reference tristimulus values in order to adjust the forward current of the LEDs, such that the difference in tristimulus values is reduced to zero. The Muthu reference however does not provide a solution for alleviating the discrepancies in the colour temperature of the desired light that are caused by the shifting of peak wavelength of the LEDs over time. In addition, the calculations required for the mathematical transformation make it difficult to implement a feedback control system with a response time that is fast enough to avoid visual flicker during dimming operations, for example.
U.S. Pat. No. 6,576,881 to Muthu et al. discloses a method and system for controlling the output light generated by red, green, and blue LEDs. Sensors positioned proximate to the LEDs to detect a first set of approximate tristimulus values of the output light. The first set of tristimulus values is communicated to a controller, which converts these values into a second set of tristimulus values representative of a standard colourimetric system. The relative luminous flux output of the LEDs is adjusted on the basis of the difference between the second set of the tristimulus values and a set of user-specified tristimulus values. The Muthu et al. reference however does not account for shifting of the peak wavelength of the LEDs due to temperature, dimming, or age of the components. In addition, the calculations required for the mathematical transformation between the two sets of tristimulus values makes it difficult to implement a feedback control system with a response time that is fast enough to avoid visual flicker during dimming operations, for example.
U.S. Pat. No. 6,630,801 to Schuurmans provides a method and system for sensing the colour point of resultant light produced by mixing coloured light from a plurality of LEDs in the RGB colours. The system comprises a feedback unit for generating feedback values corresponding to the chromaticity of the resultant light based on values obtained from filtered and unfiltered photodiodes that are responsive to the light from the LEDs, as well as a controller which adjusts the resultant light based upon the difference between the feedback values and values representative of the chromaticity of a desired resultant light. However, the method disclosed by Schuurmans does not account for shifting of the peak wavelength of the LEDs due to temperature, dimming, or age of the components.
U.S. Patent Publication No. 2003/0230991 to Muthu et al. discloses an LED-based white-light backlighting system for electronic displays. The backlighting of Muthu et al. includes a plurality of LEDs of different light colours arranged such that the combination of light colours produces white light, and a microprocessor which monitors the luminous flux, radiant flux, or tristimulus levels of the white light and controls the luminous flux and chromaticity of the white light by feedback control. The backlighting of Muthu et al. uses photodiodes with filters to determine approximate tristimulus values of the LEDs and adjust the luminous flux and chromaticity of the white light. Temperature variations from heat sinks attached to LEDs is also measured and used to account for changes in the luminous flux and chromaticity of the LEDs. Muthu et al. however, fail to consider the junction temperature during dimming of the LEDs. Muthu et al. also fail to consider the effect of peak wavelength shift and photodiode inaccuracies on the white light produced.
U.S. Pat. No. 6,441,558 also to Muthu et al. discloses a multi-colour LED-based luminaire for generating various desired light at different colour temperatures. The desired luminous flux output for each array of colour LEDs is achieved by a controller system that adjusts the current supplied to the LEDs based on the chromaticity of the desired light and the junction temperature of the LEDs. One of the shortcomings associated with the LED-based luminaire of Muthu et al. is that in order to measure the luminous flux of an array of LEDs, an optical feedback sensor is used to obtain the luminous flux from the LEDs which is communicated to the controller by a polling sequence. According to Muthu et al., the measurement sequence begins by measuring the luminous flux output of the all LED arrays in operation. Each array of LEDs is alternately switched “OFF” briefly, and a further measurement is taken. The difference between the initial measurement and the next measurement provides the light output from the LED array that was turned off. The measurement of the light output is repeated for the remaining LED arrays. A drawback of this procedure as disclosed by Muthu et al. is the excessive amount of thermal stress imposed on the LEDs during ON and OFF cycles at low frequencies.
There is therefore a need for a system and method that can effectively maintain the chromaticity, colour temperature and luminous flux of a multi-colour LED-based luminaire, while alleviating the effects of device aging and junction temperature changes on the LEDs.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.